Specification of DXIL Operations using TableGen Representation — LLVM 22.1.3 documentation

# Specification of DXIL Operations using TableGen Representation

## Introduction

DirectXShaderCompiler encapsulates, among other information, various DXIL Operations in hctdb.py. DXIL Operations are represented in one of the following two ways:

1. Using LLVM instructions.

2. Using LLVM External functions. These are represented in LLVM IR as follows:

   - “Standard” LLVM intrinsics (e.g., `llvm.sin.*`) and

   - HLSL intrinsics (defined as LLVM intrinsics in `llvm/include/llvm/IR/IntrinsicsDirectX.td`, e.g., `llvm.dx.*`)

   These are collectively referred to as LLVM Intrinsics in this note.

Following is the complete list of properties of DXIL Ops with the corresponding field name as used in `hctdb.py`. A DXIL Op is represented by a set of associated properties. These are consumed in DXIL backend passes as well as in other usage scenarios such as validation, DXIL reader, etc.

1. Properties consumed in DXIL backend passes

   1. Name of operation (`dxil_op`)

   2. A string that documents the operation (`doc`) - This is not strictly necessary but is included for readability and documentation of the operation.

   3. The generic or HLSL-specific intrinsic that maps to the operation (`llvm_name`).

   4. Unique Integer ID (`dxil_opid`)

   5. Operation Class signifying the name and function signature of the operation (`dxil_class`). This string is an integral part of the DXIL Op function name and is constructed in the format `dx.op.<class-name>.<overload-type>`. Each DXIL Op call target function name is required to conform to this format per existing contract with the driver.

   6. List of valid overload types for the operation (`oload_types`).

   7. Required DXIL Version with support for the operation.

   8. Required minimum Shader Model (`shader_model`).

   9. Minimum shader model required with translation by linker (`shader_model_translated`)

   10. List of shader stages applicable to (`shader_stages`), empty, if applicable to all stages.

   11. Memory access attributes of the operation (`fn_attr`).

   12. Boolean attributes of operation to indicate if it

       - is some kind of a derivative (`is_derivative`)

       - requires gradient calculation (`is_gradient`)

       - is a sampler feedback (`is_feedback`)

       - requires in-wave, cross-lane functionality (`is_wave`)

       - requires that all of its inputs are uniform across the wave (`requires_uniform_inputs`).

       - is a barrier operation (`is_barrier`).

## Motivation

DXIL backend passes depend on various properties of DXIL Operations. For example, `DXILOpLowering` pass will need information such as the DXIL operation an LLVM intrinsic is to be lowered to, along with valid overload and argument types etc. The TableGen file - `llvm/lib/Target/DirectX/DXIL.td` - is used to represent DXIL Operations by specifying their properties listed above. `DXIL.td` is designed to be the single source of reference of DXIL Operations primarily for the implementation of passes in DXIL backend in `llvm-project` repo - analogous to `hctdb.py` for `DirectXShadeCompiler` repo. However, the current design does not intend to encapsulate various validation rules, present in `hctdb.py`, but do not pertain to DXIL Operations. It needs to have a rich representation capabilities that TableGen backends (such as `DXILEmitter`) can rely on. Additionally, the DXIL Op specification should be easy to read and comprehend.

This note provides the design of the specification DXIL Ops as TableGen class `DXILOp` by specifying its properties identified above.

## DXIL Operation Specification

The DXIL Operation is represented using the TableGen class `DXILOp`. The DXIL operation properties are specified as fields of the `DXILOp` class as described below.

1. Each DXIL Operation is represented as a TableGen record. The name of each of the records signifies operation name.

2. A documentation string for the operation.

3. The LLVM Intrinsic that maps to the operation is represented as `Intrinsic` defined in Intrinsics.td.

4. The unique operation id is represented by an integer.

5. DXIL Operation Class is represented as follows

       // Abstraction of DXIL Operation class.
       class DXILOpClass;

   Concrete operation records, such as `unary` are defined by inheriting from `DXILOpClass`.

6. A set of type names are defined that represent return and argument types, which all inherit from `DXILOpParamType`. These represent simple types like `int32Ty`, DXIL types like `dx.types.Handle`, and a special `overloadTy` which can be any type allowed by `Overloads`, described below.

7. Operation return type is represented as a `DXILOpParamType`, and arguments are represented as a list of the same. An operation with no return value shall specify `VoidTy` as its return.

8. Valid operation overload types predicated on DXIL version are specified as a list of `Overloads` records. Representation of `Overloads` class is described in a later section.

9. Valid shader stages predicated on DXIL version are specified as a list of `Stages` records. Representation of `Stages` class is described in a later section.

10. Various attributes of the DXIL Operation are represented as a `list` of `Attributes` class records. Representation of `Attributes` class is described in a later section.

### Types specific to DXIL

Type notation used in this document viz., `<size>Ty` corresponds to TableGen records for LLVM types `llvm_<size>_ty`. Apart from `overloadTy` described above, `resRetF32Ty` is used to denote resource return type and `handleTy` is used to denote handle type.

## Specification of DXIL Operation

A DXIL Operation is represented by the following TableGen class that encapsulates the various TableGen representations of its properties described above.

    // Abstraction DXIL Operation
    class DXILOp<int opcode, DXILOpClass opclass> {
      // A short description of the operation
      string Doc = "";

      // Opcode of DXIL Operation
      int OpCode = opcode;

      // Class of DXIL Operation.
      DXILOpClass OpClass = opclass;

      // LLVM Intrinsic DXIL Operation maps to
      Intrinsic LLVMIntrinsic = ?;

      // Result type of the op.
      DXILOpParamType result;

      // List of argument types of the op. Default to 0 arguments.
      list<DXILOpParamType> arguments = [];

      // List of valid overload types predicated by DXIL version
      list<Overloads> overloads;

      // List of valid shader stages predicated by DXIL version
     list<Stages> stages;

      // List of valid attributes predicated by DXIL version
      list<Attributes> attributes = [];
    }

## Version Specification

DXIL version is used to specify various version-dependent operation properties in place of Shader Model version.

A `Version` class encapsulating `Major` and `Minor` version number is defined as follows:

    // Abstract class to represent major and minor version values
    class Version<int major, int minor> {
      int Major = major;
      int Minor = minor;
    }

Concrete representations of valid DXIL versions are defined as follows:

    // Definition of DXIL Version 1.0 - 1.8
    foreach i = 0...8 in {
      def DXIL1_#i : Version<1, i>;
    }

## Shader Stage Specification

Various shader stages such as `compute`, `pixel`, `vertex`, etc., are represented as follows

    // Shader stages
    class DXILShaderStage;

    def compute : DXILShaderStage;
    def pixel : DXILShaderStage;
    def vertex : DXILShaderStage;
    ...

## Shader Attribute Specification

Various operation memory access and boolean attributes such as `ReadNone`, `IsWave` etc., are represented as follows

    class DXILAttribute;

    def ReadOnly : DXILOpAttributes;
    def ReadNone : DXILOpAttributes;
    def IsWave : DXILOpAttributes;
    ...

## Versioned Property Specification

DXIL Operation properties such as valid overload types, shader stages and attributes are predicated on DXIL version. These are represented as list of versioned properties.

### Overload Type Specification

`overloads` field of `class DXILOp` is used to represent valid operation overloads predicated on DXIL version as list of records of the following class

    class Overloads<Version minver, list<DXILOpParamType> ols> {
      Version dxil_version = minver;
      list<DXILOpParamType> overload_types = ols;
    }

Following is an example specification of valid overload types for `DXIL1_0` and `DXIL1_2`.

    overloads = [
                  Overloads<DXIL1_0, [halfTy, floatTy]>,
                  Overloads<DXIL1_2, [halfTy, floatTy, doubleTy]>
                ];

An empty list signifies that the operation supports no overload types.

### Stages Specification

`stages` field of `class DXILOp` is used to represent valid operation stages predicated on DXIL version as list of records of the following class

    class Stages<Version minver, list<DXILShaderStage> sts> {
      Version dxil_version = minver;
      list<DXILShaderStage> shader_stages = sts;
    }

Following is an example specification of valid stages for `DXIL1_0`, `DXIL1_2`, `DXIL1_4` and `DXIL1_6`.

    stages = [
              Stages<DXIL1_0, [compute, pixel]>,
              Stages<DXIL1_2, [compute, pixel, mesh]>,
              Stages<DXIL1_4, [all_stages]>,
              Stages<DXIL1_6, [removed]>
             ];

The following two pseudo stage records in addition to standard shader stages are defined.

1. `all_stages` signifies that the operation is valid for all stages in the specified DXIL version and later.

2. `removed` signifies removal of support for the operation in the specified DXIL version and later.

A non-empty list of supported stages is required to be specified. If an operation is supported in all DXIL versions and all stages it is required to be specified as

    stages = [Stages<DXIL1_0, [all_stages]>];

### Attribute Specification

`attributes` field of `class DXILOp` is used to represent valid operation attributes predicated on DXIL version as list of records of the following class

    class Attributes<MinVersion minver, list<DXILAttribute> attrs> {
      MinVersion dxil_version = ver;
      list<DXILAttribute> attributes = attrs;
    }

Following is an example specification of valid attributes for `DXIL1_0`.

    attributes = [Attributes<DXIL1_0, [ReadNone]];

A null list of `attributes` signifies no operation attributes.

### Interpretation of Multiple Versioned Properties

Each of the versioned properties states that the specified overload type, stage or attribute records are valid for the predicated DXIL version. Only the properties corresponding to latest minimal DXIL version are applicable. Note as in the above example, any overload types, stages or attributes, that remain valid in a later DXIL version need to be specified in full. For example, consider the following specification of valid overload types:

    overloads = [
                 Overloads<DXIL1_0, [halfTy, floatTy]>,
                 Overloads<DXIL1_2, [halfTy, floatTy, doubleTy]>
                ];

It specifies that the overload types `halfTy` and `floatTy` are valid for DXIL version 1.0 and later. It also specifies that `doubleTy` is additionally supported in DXIL version 1.2 and later.

This provides the flexibility to specify properties independent of other versioned specifications in the list.

## DXIL Operation Specification Examples

Following examples illustrate the specification of some of the DXIL Ops.

`Sin` operation - an operation valid in all DXIL versions and all stages and has valid overload types predicated on DXIL version.

    def Sin : DXILOp<13, unary> {
      let Doc = "Returns sine(theta) for theta in radians.";
      let LLVMIntrinsic = int_sin;
      let result = overloadTy;
      let arguments = [overloadTy];
      let overloads = [Overloads<DXIL1_0, [halfTy, floatTy]>];
      let stages = [Stages<DXIL1_0, [all_stages]>];
      let attributes = [Attributes<DXIL1_0, [ReadNone]>];
    }

`FlattenedThreadIdInGroup` - an operation with no arguments, no overload types, and valid stages and attributes predicated by DXIL Version.

    def FlattenedThreadIdInGroup :  DXILOp<96, flattenedThreadIdInGroup> {
     let Doc = "Provides a flattened index for a given thread within a given "
               "group (SV_GroupIndex)";
     let LLVMIntrinsic = int_dx_flattened_thread_id_in_group;
     let result = i32Ty;
     let stages = [Stages<DXIL1_0, [compute, mesh, amplification, node]>];
     let attributes = [Attributes<DXIL1_0, [ReadNone]>];
    }

`RawBufferStore` - an operation with `void` return type, valid overload types predicated by DXIL Version and valid in all DXIL versions and stages.

    def RawBufferStore : DXILOp<140, rawBufferStore> {
      let Doc = "Writes to a RWByteAddressBuffer or RWStructuredBuffer.";
      let result = voidTy;
      let arguments = [dxil_resource_ty, i32Ty, i32Ty, overloadTy,
                       overloadTy, overloadTy, overloadTy, i8Ty, i32Ty];
      let overloads = [
                       Overloads<DXIL1_2, [halfTy, floatTy, i16Ty, i32Ty]>,
                       Overloads<DXIL1_3>,[halfTy, floatTy, doubleTy,
                                                    i16Ty, i32Ty, i64Ty]>
                      ];
       let stages = [Stages<DXIL1_2, all_stages>];
       let attributes = [Attributes<DXIL1_0, [ReadOnly]>];
    }

`DerivCoarseX` - an operation with no overload types and stages predicated by DXIL Version.

    def DerivCoarseX : DXILOp<83, unary> {
     let doc = "Computes the rate of change per stamp in x direction.";
     let LLVMIntrinsic = int_dx_ddx;
     let result = overloadTy;
     let arguments = [overloadTy];
     let stages = [
                    Stages<DXIL1_0, [library, pixel]>,
                    Stages<DXIL1_6, [library, pixel, amplification, compute, mesh]>
                  ];
     let attributes = [Attributes<DXIL1_0, [ReadNone]>];
    }

`CreateHandle` - an operation with no overload types, no associated `LLVMIntrinsic` and stages predicated by DXIL Version.

    def CreateHandle : DXILOp<57, createHandle> {
      let doc = "Creates the handle to a resource";
      let result = i32Ty;
      let arguments = [i8Ty, i32Ty, i32Ty, i1Ty];
      let stages = [
                    Stages<DXIL1_0, [all_stages]>,
                    Stages<DXIL1_6, [removed]
                   ];
      let attributes = [Attributes<DXIL1_0, [ReadOnly]>];
    }

`Sample` - an operation with valid overload types, stages and attributes predicated by DXIL version.

    def Sample : DXILOp<60, sample> {
      let Doc = "Samples a texture";
      let LLVMIntrinsic = int_dx_sample;
      let result = resRetF32Ty;
      let arguments = [handleTy, handleTy, floatTy, floatTy, floatTy, floatTy,
                       i32Ty, i32Ty, i32Ty, floatTy];
      let overloads = [Overloads<DXIL1_0, [halfTy, floatTy, i16Ty, i32Ty]>];
      let stages = [
                    Stages<DXIL1_0, [library, pixel]>,
                    Stages<DXIL1_6, [library, pixel, amplification, compute, mesh]>
                   ];
      let attributes = [Attributes<DXIL1_0, [ReadOnly]>];
    }

## Summary

This note sketches the design of a readable and maintainable TableGen specification of DXIL Ops in `DXIL.td` intended to serve as a single source of reference for TableGen backends (such as `DXILEmitter`) that generate C++ representations used in DXIL backend passes.